1. Field of the Invention
This invention relates to the preparation of 1,3,5-triazine carbamates which are suitable for use as crosslinking agents in curable compositions, particularly in coating compositions capable of curing at relatively low temperatures without releasing formaldehyde during cure.
2. Description of the Related Art
Various derivatives of amino-1,3,5-triazines are described in the literature as being utilized in a wide variety of fields. An important use of certain of these derivatives, such as the methoxymethyl derivatives of melamine and guanamine, is as crosslinkers and/or reactive modifiers in curable compositions which contain resins having active hydrogen groups. While these methoxymethyl derivatives provide excellent results in a number of aspects, they also have the disadvantage of releasing formaldehyde as a volatile by-product under curing conditions. It has long been a desire of industry to find acceptable alternatives which do not emit formaldehyde during cure.
One such non-formaldehyde alternative that has shown great promise is the class of isocyanate and carbamate-functional 1,3,5-triazine crosslinking agents disclosed in commonly owned U.S. Pat. Nos. 4,939,213, 5,084,541 and 5,288,865, EP-A-0604922 (corresponding to U.S. application Ser. No. 07/998,313, filed Dec. 29, 1992), EP-A-0624577 (corresponding to U.S. application Ser. No. 08/061,905, filed May 14, 1993), U.S. application Ser. No. 08/138,581 (filed Oct. 15, 1993), U.S. application Ser. No. 08/239,009 (filed May 6, 1994), U.S. application Ser. No. 08/286,835 (filed Aug. 5, 1994), U.S. application Ser. No. 08/324,549 (filed Oct. 18, 1994) and U.S. application Ser. No. 08/08/705,472 (filed Aug. 29, 1996), which are incorporated by reference herein for all purposes as if fully set forth. Specifically, the carbamate functional 1,3,5-triazines disclosed in these references have been found to be particularly useful as crosslinkers in coating compositions based upon hydroxy functional resins, with the cured coatings possessing a wide range of desirable properties.
Other non-formaldehyde emitting alternatives include, for example, the class of lactam substituted 1,3,5-triazine crosslinking agents disclosed in commonly owned WO 93/10117 (corresponding to U.S. application Ser. No. 07/973,676, filed Nov. 9, 1992) and the class of acetal and enamine functional 1,3,5-triazine crosslinking agents disclosed in commonly owned U.S. application Ser. No. 08/408,323 (filed Mar. 21, 1995), which are also incorporated by reference herein for all purposes as if fully set forth.
While some of these these non-formaldehyde emitting systems have shown great promise, their preparation methods can be somewhat cumbersome, difficult and expensive. For example, in previously incorporated U.S. Pat. No. 4,939,213 and U.S. Pat. No. 5,084,541, the 1,3,5-triazine carbamates are produced in a two-step process by first reacting an amino-1,3,5-triazine with oxalyl chloride to produce an isocyanate functional intermediate, then reacting this intermediate with an alcohol. Further, in previously incorporated U.S. Pat. No. 5,288,865, carbamate functional 1,3,5-triazines are produced in a one-step process by reacting a haloamino-1,3,5-triazine with an acid halide. The primary disadvantage of these processes include the use of certain costly halogenated starting materials, production of substantial amounts of halogenated by-products, and low ultimate yield of the desired products. Therefore, the search continues for simple and economical routes to preparing non-formaldehyde crosslinkers.
Some of the problems with these processes have been solved by the process disclosed in previously incorporated U.S. application Ser. No. 08/061,905, wherein carbamate functional 1,3,5-triazines are produced by reacting an at least bis-amino 1,3,5-triazine with an acyclic organic carbonate in the presence of a strong base. Disadavantages to this process include, for example, that the strong base must be neutralized to remove it from the end product and that certain aspects of the reaction must be carefully controlled in order to avoid color in the end product.
In regard to the production of mono-carbamates, Matsui and Otaguro, 1976, Kogyo Kagaku Zasshi, 67: 12, pp. 2402-2403 describe the formation of a mono-substituted product, 2-(ethoxycarbonylamino)-4,6-dichloro-1,3,5-triazine, from the reaction of cyanuric chloride with potassium cyanate. However, the attainment of di-substituted derivatives are not therein disclosed.
It has now been surprisingly discovered that di- and more highly substituted 1,3,5-triazine carbamate crosslinkers can be produced by reacting, in an organic solvent reaction medium, (i) a 1,3,5-triazine derivative having thereon at least two cyanate displacable leaving groups and (ii) a cyanate-containing reagent, in the presence of (iii) an isocyanate-reactive compound, such as a hydroxy-functional compound. The process of the present invention advantageously does not require any costly starting materials. Furthermore, the 1,3,5-triazine carbamate crosslinkers prepared by the present invention's process can be produced directly, without handling of the 1,3,5-triazine intermediates. When employed in coating compositions, the 1,3,5-triazine carbamate crosslinkers produced by the present invention's process produce films which have good appearance, solvent resistance and gloss characteristics.